Lego City is a fabulous game. It's got a great script, great levels, HUGE open world with loads of things to find and some fun uses of the WiiU controller - like holding it up to scan the room.
As for if it's worth the whole machine ... probably not. No one game is worth a console to me, but there are plenty of other great games you can get. If the second hand game market is strong where you are, I'd say go for it.
I don't know what you mean by WiiU mini either - there's a Wii mini but this is not the WiiU, it's the Wii. The WiiU has the controller with the television screen on. It's also backwards compatible so it will play Wii games as long as you have a Wiimote.